Many electronic devices such as computers, VCR's, and digital cameras use magnetic storage as a form of nonvolatile memory. Different methods of magnetic storage include magnetic tape (used in VCR's and digital backup tapes for example) and rigid magnetic media (used in hard disk drives for example). The read portion of the read/write head used in magnetic storage devices generally use the principle of magnetoresistance and can be constructed in a variety of ways. Anisotropic magnetoresistance (AMR), giant magnetoresistance (GMR), and magnetic tunnel junction (MTJ) read sensors can all be used as magnetic read sensors and are well known in the art.
FIG. 1 shows a prior art GMR sensor. The hard bias layer 10 has a thickness 22 and may be made out of materials such as CoPtCr. A ferromagnetic free layer 14, usually made of NiFe, is on top of a nonmagnetic metallic layer 16, usually made of Cu. Under the metallic layer 16 is a ferromagnetic pinned layer 18, which has a magnetic orientation that is pinned by an antiferromagnetic layer 20. The ferromagnetic pinned layer 18 is usually made of CoFe. The antiferromagnetic layer 20 is usually made of PtMn. Under both the hard bias layer 10 and antiferromagnetic layer 20 is a dielectric gap layer 19 followed by the substrate 28. The substrate 28 is usually made of an alloy of Al2O3—TiC composite (N-58). Electric current 26 flows from electrode 25A through the read sensor along the plane of the free layer 14 to electrode 25B, and, therefore, the sensor is referred to as a current-in-plane (CIP) sensor. Suitable materials for the electrodes 25 A,B include Rh and Au. Changing the magnetic orientation of the free layer 14 by exposing the read sensor to a magnetic media changes the resistance to current 26 flowing through the read sensor. Data on the magnetic media can be read by detecting changes in current 26 flow as the read sensor passes over the magnetic media.
FIG. 2 shows one half a prior art ultra contiguous junction (UCJ) magnetic read sensor. The UCJ sensor has a hard bias layer 10 with a thickness 22 that is positioned such that its geometrical center is collinear with a ferromagnetic free layer 14. The hard bias layer 10 is grown on a seed layer 12 that has a thickness 24. The ferromagnetic free layer 14 is next to a nonmagnetic metallic layer 16. An antiferromagnetic layer 20 determines the magnetic orientation of the ferromagnetic pinned layer 18. The seed layer 12 and antiferromagnetic layer 20 are built on a dielectric gap layer, which is in contact with substrate. Both the dielectric gap layer and substrate are not shown but are well known in the art. The UCJ geometry is desirable to further increase the storage density of magnetic recording devices. As the bit size area is decreased in order to increase the areal density, the free layer 14 dimensions need to be reduced to match the size of the recorded magnetic domains. Furthermore, for optimum stabilization of the free layer by the hard bias layer, the ratio and thus the physical thicknesses of these two layers need to be similar. Hence, the UCJ geometry readily allows matching of the physical dimensions of the hard bias and the free layers in the magnetic read sensor.
The hard bias layer 10 orients the magnetic axis of the free layer 14 when the free layer 14 is not subject to an external magnetic field that would come from the magnetic flux from the magnetic recording media, which the magnetic read sensor would detect when the read head flies over the magnetic media. Failure to provide a biasing structure such as a hard bias layer 10 can result in Barkhausen noise. Barkhausen noise reduces the signal to noise ratio of the read sensor and makes it difficult to increase the storage density on the magnetic media. Inadequate biasing can also result from local variations of the magnetization direction at the junction between the hard bias layer 10 and the free layer 14. These variations may result in head instability, current amplitude changes, and non-reproducible performance from device to device.
Magnetic heads used for ultrahigh density magnetic storage are more susceptible to biasing problems, because ultrahigh density magnetic heads use of very thin hard bias layers 10. Consequently, for very thin hard bias layers 10, there are a very limited number of magnetic grains at the junction between the hard bias layer 10 and the free layer 14.
Materials utilized for read sensors are polycrystalline in nature. Therefore, the magnetization direction of the individual grains can differ from grain to grain. This can result in magnetically unstable heads. The problem is further exacerbated by the superparamagnetic effect, whereby the magnetic orientation of progressively smaller magnetic grains can be randomized by thermal fluctuations.
Biasing layers currently used in recording devices exhibit, at best, two dimensional random orientation of the magnetization of the individual crystallites. The degree of alignment in the hard bias layer 10 is increased by employing an external setting field. However, due to the aforementioned thermal and microstructural effects, the magnetization direction, upon removal of the setting field, can significantly relax from the desired orientation. This is particularly detrimental for the grains at the boundary between the hard bias 10 and the free layer 14.
FIGS. 3A and B show an example of two dimensional random orientation of the magnetic axes of the grains in the hard bias layer 10. FIG. 3A shows a top view of a hard bias layer 10 with magnetic axes 30 of grains that are randomly oriented relative to the normal of the thin film plane comprising the hard bias layer 10. FIG. 3B shows a side view of the hard bias layer 10 of FIG. 3A. The side view shows that the net magnetization of the thin film is predominantly parallel to the film plane. However, as shown in FIG. 3A, the magnetic axes 30 of grains can vary from canted to perpendicular as seen in the top view. To achieve full confinement of the magnetization in the plane of the hard bias materials is not easily accomplished.
While current magnetic read heads typically have only two dimensional random orientation of the magnetization of the individual crystallites, magnetic recording media (a hard disk for example) can be manufactured such it exhibits uniaxial magnetic anisotropy (commonly known as magnetic “Orientation”). “Orientation” is accomplished when suitable growth conditions are employed so that the crystallites comprising the magnetic material exhibit a strong orientation of their magnetic axis along the desired direction.
FIG. 4 shows an example of a hexagonal close pack (HCP) cell with the [11 20] plane indicated with dashed lines. In the case of HCP Co-alloys, the magnetic axes of the crystallites are parallel to the plane that is orthogonal to the basal planes of the hexagonal unit cell (i.e. the magnetic axes are parallel to the [11 20] plane).
FIG. 5 shows an example of a body centered cubic (BCC) crystalline structure. Atoms reside at each corner of the cube and also in the middle of the cube.
In magnetic media, “Orientation” is typically achieved by growing the magnetic alloy on suitable underlayers grown in turn on substrates which are circumferentially textured. U.S. Pat. No. 5,989,674 describes the influence that a textured substrate has on the growth of an underlayer structure. When deposited on a properly textured substrate, the lattice parameters of the BCC Cr and Cr-alloys along the radial direction are greater than the corresponding lattice parameters along the track direction. This anisotropic strain relaxation leads to a better lattice match of the [11 20] prismatic plane of the HCP Co-alloy, which is deposited on top of the underlayer, along the track direction. Consequently, the magnetic axis of a crystallite predominantly aligns along the track direction. “Orientation” requires that the Co-alloy crystallites grow with their [11 20] plane parallel to the substrate. To ensure this, the underlayer material is grown with its [002] plane parallel to the substrate plane.
FIG. 6 shows an example of the epitaxial relationships between a HCP magnetic layer and a BCC CrMo layer. BCC atoms 32 are shown on a BCC lattice 36. The [002] plane is shown parallel to the page. The four BCC atoms 32 are not from one crystalline cell, but rather are from specific corners of four adjoining BCC cells. Four HCP atoms 34 comprise the [11 20] plane 38 of the HCP structure.
Current methods in recording media manufacturing to render the underlayer [002] plane parallel to the film plane use high temperature growth (typically greater than 200° C.) and sputter deposition. High temperature growth is not always desirable or possible for the fabrication of magnetic read sensors. Currently, high temperature growth will damage other components of the read head, if attempted. Thus, current methods of underlayer and bias layer growth are not suitable for the creation of uniaxial magnetic anisotropy in magnetic read heads.
Given the need for ever increasing storage densities, there is a need to increase the magnetic thermal stability and degree of orientation of the magnetization in hard bias materials for high density recording.